How do I combine an animation with two different fps'?
The NLA Editor
Could consider dropping the actions into the NLA editor and scaling them to match.
For example sake have two actions 100 frames of an 100 frame per second action  and 24 frames of a 24 frame per second action. At there designated frame rates each would last one second.
As noted in question if scene frame rate is 24 FPS the 100 frame action goes for 100 / 24 (approx 4 seconds), or conversely if at 100 fps the 24 frame per second lasts 24 / 100 seconds.

Among a few ways to animate time in the NLA editor can simply scale the length of one to match the other.
Drive the strip scale.
Better still can add a driver using the scene frame rate
Setting up the driver, right click in the scale property box and choose add driver

Add a single property variable, name it &quot;fps&quot; and point it to scene.render.fps by choosing scene and crunching render.fps as the data path. Now for the 24 frame per second action make the driver expression
24 / fps

copy the driver, paste into the 100 FPS strip scale property. Change the expression to
100 / fps

Now the two strips will match length, eg at 24 fps the 24 fps is 24 frames long and taking up 1 second, as is the 100 fps and vice versa.

Please note, could associate an fps with the action and use it in driver expression.  Also for the correct fps the scene.render.frame_base property should be taken into account, eg at 23.98 fps the fps is 24 and the base is 1.001, ie actual frame rate = scene.render.fps / scene.render.fps_base.